Visual motion processes were studied with luminance- and contrast-modulated gratings. A sine-wave luminance grating was displaced abruptly back and forth by 3/16 cycle. The display sequence is ambiguous in that each 3/16-cycle phase shift (short-path motion) could just as readily be seen as a 13/16-cycle shift (long-path motion) in the opposite direction. By varying the duration of the interval (IFI) between the two phase positions, the luminance of the IFI, or the spatial frequency of the grating, it was possible to bias the ambiguous percept in favor of short-path motions or long-path motions. A contrast-modulated grating displaced through 3/16 cycle always appeared t undergo short-path motion. Current motion models which incorporate Reichardt-type/energy mechanisms and certain types of auxiliary signal transformations which precede those mechanisms do not adequately explain the effects of IFI intensity on the perceived motion of a sinusoidal grating or the effect of IFI duration on the perceived motion of a contrast-modulated grating.